Posted at - 23th Dec 2018
Radio Waves and Crystal Radio
It was a long time ago, when my beloved grandfather told me about galena radios, a type of radio receiver which works without any additional electrical power, but only powered from the radio wave itself. Being just a child I was confused. My grandfather explained to me that the radio waves themselves are electric waves by their nature, therefore if I will throw some wires into trees they will be able to pick a small sample from radio waves and I could listen to them just using a galena crystal in a matchbox and a pair of headphones.
Of course, I didn't understood too clearly how this could work, but now, after about 25 years since then I've made a crystal radio just as a proof of concept in order to show and explain to others how this works and what a radio wave is.
Brief Overview of Radio Waves
In the image above is depicted the simplest antenna, a dipole. A dipole consists of an open ended conducting wire fed at its center forming two equal elements in this way, the two thick elements in the image.
The Transmitter Example
The transmitter is nothing more than an electric generator, it can even be replaced with a simple battery for the sake of this example. When the battery is connected to the dipole antenna, some of the electrons of one element are sucked and pushed by the battery to the other element, through battery itself. Then the two elements are charged like a capacitor (the wire are the plates, and the air is the dielectric) and then nothing happens. If the battery voltage is further increased, then more electrons are being displaced from one element to another, creating a more powerful electric field around, having one element positive (with less electrons) and one negative element (with many more electrons). Also, when electrons are moving through their path (wire element) they generate electric current.
Long story short, when a voltage force is applied between elements of the dipole electrons starts moving from one element to another measured as current, when further electrons cannot be displaced then the electric current reach its minimum while the electric field - voltage between elements - is at maximum because now the electrons are standing still, more in one element and less in the other.
Of course, these things happens at the speeds close to speed of light, therefore are not easy to be measured, also more things occurs (magnetic field out of phase with electric field, near vs. far fields, wave reflection depending to length of electron travel as a function of frequency, dipole length and speed light, etc, etc), but let's stay simple for the sake of example.
Now if the polarity of the battery is suddenly reversed, the electrons starts to flow again, now from the element with many electrons, sucked and pushed to the another element through the battery itself, again creating current and electric field. Then, again nothing happens once the dipole is charged, because to radiate power the electrons need to move continuously back and forth within dipole elements.
It can be seen that the battery should change again its polarity in order to keep the generation of current and electric fields over and over again. If the battery is then reversed the effect take place again. The number of polarity changes per second can be analogous as the transmitter frequency.
Since the battery is a continuous electric source it cannot switch the polarity by itself in a regular way. To keep these phenomenon going we need to fed the antenna with an alternative voltage with a given frequency, let's say an alternator of 100KHz.
Now we have an antenna with electrons forced to flow from one element to another 100,000 times per second creating an electric field changing at a rate of 100,000 times per second. The dipole antenna generates a stronger electric field nearby than magnetic field, other atennas are creating stronger magnetic field, other types claim to produce both.
It is important to know that when an electron or a charged particle as it moves it generates traces of energy which in turn is interacting with the matter at atomic level. That energy is composed from an electric field and a magnetic field. If both fields are in phase, when the electric field is varying the magnetic field is also varying and when the magnetic field is varying the electric field is also varying, thus is sustained by itself over long dinstances, traveling close to the speed of light depending of the path resistance, that's electromagnetic radiation. Also, important to note that "sustained by itself" in other words can be thought as the energy traveling away from antenna will still travel by itself even if the transmitter was shutted down.
The radio waves are classified as a small portion of the electromagnetic spectrum beside visible and invisible light, UV, X-rays, Gamma Rays, etc.
The Receiver Example
While electromagnetic energy is traveling around it is impacting any other atoms and charged particles nearby. If a dipole antenna is connected to a receiver (or shorted at its center) when the electromagnetic wave hits the dipole the electrons are forced to replicate the vibration of the others in transmitter antenna. Thus, they are displaced from one element to another through the receiver, creating a current direct proportional with power of the received electromagnetic radiation at the frequency of the transmitter and inverse proportional with the distance from the transmitting antenna. This is true not only for receiving antennas but for any matter objects like metals, trees, living tissue, rocks, water, gases, etc.
Now, since the current generated in the receiving dipole passes between its elements through the receiver it can be measured, amplified, decoded, etc. by the receiver.
Also, there are many many things to take into consideration like antenna gain, swr, impedance matching, phase, common current, path loss, wave reflection, refraction, diffraction, polarisation and much more, but I'm not going to write an antenna book now :-)
The Crystal Receiver
The antenna is a crucial component of a radio setup because it should be able to develop big power levels in order to drive headphones without any amplification. Here can be enumerated the following criteria: low loss cables, tick wires, good ground connection/radials, enough length (depending the broadcast band targeted).
The receiver setup must have a matching circuit to match the antenna impedance ensuring maximum power transfer, sharp band pass filters with low attenuation (high Q) using variable air-capacitors, low loss coils, adjustable electric or inductive coupling factor, notch filters to null unwanted stations, sensitive detector (biased, low capacitance, balanced, etc), sensitive headphones correctly matched to the rest of circuit.
Here is the schematic of my receiver:
The receiver consists from two tuned LC tanks mutually coupled together, an optionally biased germanium diode, a 100K resistor used as DC return path (because piezoelectric earpices are capacitive so there is no sufficient DC return path for strong signals even with 100K connected), a smoothing capacitor for high impedant headphones.
The biased detector was designed to have an adjustable voltage and medium impedance source to set the bias at the knee of diode conduction. The bias is not mandatory for a germanium diode. It doesn't make any big difference when is biased in a high impedant load. The bias is more likely needed for silicon diodes though.
I'll not try to describe here how the diode AM detector works because the internet is full with good explanations.
The audio impedance matching is made automatically when tune the coils since the piezoelectric earpieces are high impedant in their nature lightly loading the tuned tank. A switch on the headphone line switch the earpieces in series or in parallel configuration. As an information, the sensitivity is increased when the earpeces are connected in series.
Several drawbacks and flaws of this receiver:
- the audio impedance matching is made correctly only when the second tuned tank LC2 is set to resonance using maximum inductance and lower capacitance. Otherwise, the voltage is lower and the audio volume is also reduced.
- there is no matching circuit between antenna and receiver, the matching is made only from coils turn ratio in the limit of tuning using few turns in LC1 and many turns in LC2, sometimes this method is not enough.
- when tuning the LC tanks using only few turns, the rest of the coil is acting as an auto transformator, the primary represented by the first few turns and the secondary by rest of unused turns, steping up the voltage and interacting with environment silently lowering the Q factor of the coil. So a good solution here could be disconnecting the remaining unused turns.
- a capacitor should be placed in parallel with the biasing source of the detector in order to lower the impedance of the biasing source if it is used.
The tuning steps of the LC2 are:
1. 40MHz to 23MHz
2. 22MHz to 12MHz
3. 12MHz to 7MHz
4. 7MHz to 4MHz
5. 4.5MHz to 2.5MHz
6. 2.8 to 1.7MHz
Using a ferrite rod the range is pulled down to approx 500KHz.
Despite all drawbacks above, this receiver is able to pick AM transmitters from hundred (MW) or thousand (SW) of miles away at night using 15 meters of horizontal thick copper wire at 10m above the ground and a water pipe ground connection. From Bucharest/Romania I'm able to hear stations from Greece, Ukraine, Hungary, Turkey, etc. in MW band at night and China, Korea, UK, Turkey, etc. in SW band.
It's always a pleasure to listen to this radio ;-)
73 de YO3BN
